The present invention relates to a food preparation device including a toaster, an oven, and a food warmer, having a smart spring. More particularly, although not exclusively, the invention relates to a toaster/oven/food warmer having a spring or other deformable member made of Shape Memory Alloy (SMA) metal.
Known toasters for example include a carriage that is moved vertically or horizontally against the tension of a spring. In the heating position, the carriage is held down or back by a catch. At the end of the selected heating time, the catch is released to in turn release the carriage which returns under spring force to deliver the food item to a retrieval position. A number of different mechanisms have been proposed, but they are all fairly complex in design and unnecessarily expensive to produce.
Also, the type of mechanism described above releases the spring suddenly so that the carriage returns to the retrieval position with a jolt. This can be noisy and can sometimes result in the food being catapulted to the floor.
It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages and/or more generally to provide an improved toaster having a smart spring.
There is disclosed herein a food preparation device comprising:
a chassis,
a heating element supported by the chassis,
a carriage for directly or indirectly supporting a food item, the carriage being movable with respect to the chassis to move the food item to/from a cooking position adjacent to the heating element,
an SMA resistance member connected to the carriage and the chassis, and
electric/electronic circuitry controlling voltage and/or current to the SMA resistance member to selectively heat the resistance member and alter its shape to move the carriage to/from the cooking position.
Preferably, the resistance member is in the form of a coil spring.
When an SMA material is heated to its transformation temperature, it will take on its memorised shape or length. In the present invention, the memorised shape of the resistance member is typically of contracted length.
Preferably, when heated, the SMA resistance member is of a contracted length.
Preferably the carriage is at a food item-retrieval position when the SMA resistance member is at its heated and contracted length.
As an alternative, the SMA resistance member is in the form of a length of wire riding along a number of pulley wheels mounted directly or indirectly to the chassis.
Preferably, the electric/electronic circuitry provides a pulsed, or otherwise controlled voltage/current to the SMA resistance member so as not to burn-out the resistance member when it is contracted.
There is further disclosed herein a method of altering the state of an SMA resistance member of the method comprising applying a pulse of high voltage/current to the member so as to quickly increase its temperature to a transition temperature and thereafter applying less voltage/current, either by reduced continuous current or pulses of voltage/current, to maintain the temperature at or slightly higher than the transformation temperature.